The present invention relates to a new and improved construction of a reference value transmitter for a phase clipping or phase cutting control for starting a squirrel cage-asynchronous motor, comprising a start-up regulator for the upward regulation of a reference value transmitter-output voltage from a start-up voltage value which is constant during a primary start-up phase to a rated voltage value with a predetermined build-up speed during a terminal start-up phase.
As far as the state-of-the-art starters for squirrel cage-asynchronous motors is concerned the simpliest as concerns expenditure of equipment is the star-delta reversing switch or switching arrangement. The motor is started-up by means of a stator winding which is connected in a star circuit configuration and after reaching the stable rotational speed the stator winding is switched into a delta circuit configuration. In the case of star-connected stator windings the motor torque must be sufficiently above the load torque and switching must not take place prior to termination of the start-up operation. The reduced torque or rotational moment of the motor connected in star circuit configuration, which requires a corresponding counter moment of the driven machine or load, the occurrence of current surges which are damaging to the windings, especially when switching-over from the star circuit configuration to the delta circuit configuration, and the occurrence of torque surges during the switching operation, which can be particularly considerable in the case of operating machines possessing small moment of inertia constitute the essential drawbacks of a star-delta switching arrangement; but in contrast thereto such have the advantage of relatively low equipment expenditure.
There are known to the art a number of further starting techniques for squirrel cage-asynchronous motors, the greatest technological advantages of which are considered to be simultaneous frequency- and voltage regulation. With these techniques there are employed controlled power semi-conductor-switching elements in the motor supply lines and there are used tachodynamos for obtaining signals representing the rotational speed of the motor. Such techniques render possible, in each instance, practically ideal starting-up of the motor, but the expenditure in hardware needed to do so is quite considerable. Less complicated and expensive are phase clipping-contols operating with a fixed frequency of the supply voltage, for instance with current limiting or voltage setting or positioning. With the current limiting technique the motor current, during starting-up of the motor, is maintained constant at a predetermined fraction of the starting current which flows during direct switching-on of the motor. However, at the end of the starting phase the start-up or starting torque steeply increases, leading to a pronounced acceleration which, for some driven machinery, can be impermissibly great. When starting-up a motor while using the voltage setting technique the stator voltage is maintained constant at a start-up voltage value which is below the rated voltage during a primary start-up phase during which the motor should start-up as closely as possible to the rated rotational speed, and then during a terminal start-up phase such stator voltage is upwardly regulated to the rated voltage value. The primary start-up phase and the terminal start-up phase collectively constitute the start-up time of the drive system. The start of the build-up or upward regulation, i.e., the start of the terminal start-up phase is thus normally controlled as a function of time. A predetermined time-span is set for the primary start-up phase, and upon expiration thereof there automatically begins the build-up or upward regulation. Consequently, owing to the constant terminal start-up phase there is also fixed the start-up time during which the motor should have reached its rated rotational speed. However, even with proper setting of the time operational disturbances can arise if the operating conditions change during the start-up operation owing to certain factors, such as for instance if the motor is subjected to fluctuating loads, so that the motor starts-up more rapidly or slowly than under normal conditions and, consequently, the upward regulation occurs to early or too late. Because of too early initiation of the build-up or upward regulation there are formed excess currents and pronounced accelerations, and when the build-up or upward regulation occurs too late the motor remains fixed at lower rotational speeds. The reduced expenditure in equipment which is possible in the case of startors with phase clipping or phase cutting control obtained by means of such current limiting or voltage setting is accordingly obtained at the expense of functional drawbacks and also at the expense of a more difficult manipulation or operation of such starter due to errors which easily can arise during the use and operation thereof.